A computer device such as an information processing device or a storage device is coupled to a normal power supply device and a backup power supply device such as an uninterruptible power supply in order to inhibit a system operation from being stopped due to the stop of an external power supply such as a commercial power supply and inhibit data from being lost. A backup power supply device of this type includes a detecting circuit (detector) and a switching circuit (switch). The detecting circuit detects a reduction in a power supply voltage to be supplied to the computer device. The switching circuit is used to supply a power supply voltage from a battery to the computer device in response to the detection of the reduction in the power supply voltage by the detecting circuit. Japanese Laid-open Patent Publication No. 2003-79069 is disclosed as related art, for example.